1. Field of the Invention
The present invention relates to a zoom lens and an optical apparatus using the zoom lens, and, more particularly, to a zoom lens that is suitable for use in an optical apparatus, such as a photographic camera, a video camera, a digital camera, or a broadcasting camera, in which aberrations, particularly chromatic aberration, have been properly corrected by using a diffraction optical surface in a portion of a lens system.
2. Description of the Related Art
In recent years, smaller and lighter optical apparatuses, such as home videos and digital cameras, have caused a demand for an image pickup zoom lens device, used in such optical apparatuses, that has a small overall length and whose front lens unit has a small diameter.
What is called a rear focus zoom lens device that performs focusing as a result of moving a lens unit other than a first lens unit disposed at an object side is known as one means for achieving this object.
In general, the first lens unit of the rear focus zoom lens device has a smaller effective diameter than the first lens unit of a zoom lens device that performs focusing as a result of moving the first lens unit. Therefore, it becomes easier to reduce the size of the entire lens system, and to perform a shooting operation at a close distance, particularly at a very close distance. In addition, since a small, light lens unit is moved, only a small driving force needs to be exerted on the lens unit, so that, for example, proper focus can be quickly obtained.
Such a rear focus zoom lens device is disclosed in, for example, Japanese Laid-Open Nos. 62-24213 and 62-247316. The rear focus zoom lens devices disclosed in these documents comprise four lens units, which are, in order of lens units from the lens unit closest to the object side, a first lens unit having a positive refractive power, a second lens unit having a negative refractive power, a third lens unit having a positive refractive power, and a fourth lens unit having a positive refractive power. The second lens unit is moved to change magnification, and the fourth lens unit is moved to perform focusing and to change an image plane with changes in magnification.
In general, in order to effectively increase mountability to a camera when the camera is not used, the lenses are collapsibly mounted therein. However, in the zoom lens device of the above-described type whose second lens unit takes over most of the magnification change function for changing magnification, sensitivities of the first and second lens units with respect to decentering are too large, so that this type of zoom lens device is not suited to be collapsibly mounted to the camera.
To overcome this problem, a zoom lens device having a magnification change ratio of the order of 3 has been proposed in Japanese Patent Laid-Open No. 10-62687 (U.S. Pat. No. 6,016,228). This zoom lens device comprises four lens units, which are, in order of lens units from the lens unit closest to the object side, a first lens unit having a positive refractive power, a second lens unit having a negative refractive power, a third lens unit having a positive refractive power, and a fourth lens unit having a positive refractive power. The first lens unit consists of a single lens, and the second, third, and fourth lens units are moved to perform magnification changes. The fourth lens unit is moved to perform focusing. Therefore, the optical system is simplified, thereby allowing it to have a structure that is suitable for collapsible mounting.
On the other hand, in recent years, a method that uses a diffraction optical element (diffraction optical surface) has been proposed as a method of restricting the occurrence of chromatic aberration.
For example, in Japanese Patent Laid-Open Nos. 4-213421 (U.S. Pat. No. 5,044,706) and 6-324262 (U.S. Pat. No. 5,790,321), chromatic aberration is reduced by using a diffraction optical element for the single lens. In U.S. Pat. No. 5,268,790, the use of a diffraction optical element in the second lens unit or the third lens unit of the zoom lens device is proposed in order to reduce the number of lenses used and the size of the zoom lens device compared to conventional zoom lens devices. However, the reduction in the number of lenses and the reduction in the size of the zoom lens device have not been satisfactorily achieved.
The zoom lens devices disclosed in Japanese Patent Laid-Open Nos. 9-211329 (U.S. Pat. No. 5,872,658) and 11-271616 (U.S. Pat. No. 6,069,743) each comprise four lens units, which are, in order of lens units from the lens unit closest to the object side, a first lens unit having a positive refractive power, a second lens unit having a negative refractive power, a third lens unit having a positive refractive power, and a fourth lens unit having a positive refractive power. The first lens unit consists of a single lens, and a diffraction optical surface is used in the first lens unit in order to reduce the number of lenses used and to reduce the size of the zoom lens device.
When, in order to reduce the size of the optical system, an attempt is made to reduce the number of lenses used by merely increasing the refractive power of the lens units, the thicknesses of the lenses increase. Therefore, the size of the optical system is not satisfactorily effectively reduced and, at the same time, it becomes difficult to correct various aberrations.
When an attempt is made to collapsibly mount the lenses when they are not used, mechanical structural errors, such as lens tilting, inevitably become large, so that, when lens sensitivity (that is, the ratio of the amount of displacement of an image plane to the amount of displacement per lens) becomes large, optical performance deteriorates and image jitters occur when magnifications change. Therefore, it is desirable that the sensitivities of lens units be kept as small as possible.
In zoom lenses comprising four lens units, which are a lens unit having a positive refractive power, a lens unit having a negative refractive power, a lens unit having a positive refractive power, and a lens unit having a positive refractive power, when an attempt is made to perform magnification changes by moving only the second lens unit and the fourth lens unit, most of the task of changing magnification must be performed by the second lens unit, so that the refractive powers of the first and second lens units inevitably need to be made large.
In contrast, the zoom optical system disclosed in Japanese Patent Laid-Open No. 10-62687 is suited to be formed into a collapsible mount structure because the sensitivities of the first and second lens units become relatively small. However, since the first lens unit is fixed when magnification is being changed, it is difficult to obtain a high magnification change ratio equal to or greater than 5.
In zoom lens devices having a high zoom ratio equal to or greater than 5, it is difficult to properly correct changes in chromatic aberration that occur with changes in magnification unless chromatic aberration that occurs in each of the lens units is corrected to a certain extent. Although the number of lenses used can be reduced using aspherical surfaces in the optical system, when an attempt is made to reduce the number of lenses used by merely using aspherical surfaces, the refractive powers of the positive lenses become too large, so that the lenses need to be formed with shapes that are difficult to manufacture. Therefore, the refractive power at a magnification change portion needs to be weakened, thereby making it difficult to reduce the overall length of the zoom lens device.
In order to overcome these problems, various zoom lens devices that include diffraction optical surfaces have been proposed. However, in order to obtain a high-performance optical system that is adaptable to, for example, digital still cameras having two million or more pixels by using a magnification change ratio of the order of 5 and a simple structure, it is necessary to properly set the lens structure.
It is an object of the present invention to provide a zoom lens which comprises a diffraction optical surface in a portion of an optical system in order to reduce chromatic aberration that occurs in each lens unit by combining diffractive optical action and achromatization effect of a diffractive system, which has reduced overall length as a result of reducing the number of lenses used while the refractive power at a magnification change portion is maintained, and which provides good optical performance over the entire magnification change range from a wide angle end and to a telephoto end. It is also an object of the present invention to provide an optical apparatus using the zoom lens.
To these ends, according to the present invention, there is provided a zoom lens that includes, in order from an object side to an image side, a first lens unit consisting of a positive lens element and having a positive optical power (equal to the reciprocal of the focal length), a second lens unit having a negative optical power, a third lens unit having a positive optical power, and a fourth lens unit having a positive optical power. In the zoom lens, when zooming is performed from a wide angle end to a telephoto end, the first lens unit and the third lens unit are moved so that, at the telephoto end, they are positioned closer to the object side than to the wide angle end, and the second lens unit is moved so that, at the telephoto end, it is positioned closer to the image side than to the wide angle end. The zoom lens also includes at least one diffraction optical surface.
Further objects, features and advantages of the present invention will become apparent from the following description of the preferred embodiments with reference to the attached drawings.